A geographical area covered by a cellular wireless communications system may be separated into a patchwork of smaller geographical areas or cells, which are each served by a servicing base station. Each base station communicates with end user terminals, which are located within the cell served by the base station. In a conventional cellular wireless communication system, an end user terminal, which may be a mobile or a stationary end user terminal, attempts to communicate directly with a servicing base station, with no other terminals being involved. Some of the end user terminals may be located unfavourably within the cell so that a direct communication with the base station is not possible, or will only support a low data rate. For example, the end user terminal may be located at an edge of the cell serviced by the base station or may be in a position with respect to the base station subject to a deep lognormal fade.
This problem has been addressed in a multi-hop wireless communications system in which the end user terminal and the servicing base station may communicate with each other, either directly and/or via at least one of a plurality of relay equipments. Therefore, if a direct communication between a source equipment (for example, a base station or an end user terminal) and a destination equipment (for example, the other of an end user terminal or base station, respectively) is poor, an indirect communication can be made via one or more relays. If good propagation conditions exist along the path of the indirect communication, higher data rates can then be supported. Also, because the source equipment and any relay equipment on the communication path will generally transmit at a lower power as compared to the power with which the source equipment would have to transmit in order to successfully communicate over a direct path, interference within the communication system can be reduced. The plurality of relay equipments may include dedicated relay equipments, which are either fixed or mobile and which are deployed either by the system operator or by the user of the end user terminal, to enhance coverage in a particular area of a system. The plurality of relay equipments may also include other end user terminals with the functionality to act as a relay equipment, and again may be fixed or mobile.
A relay equipment receives a radio signal from a source equipment and performs some further processing on it, before retransmitting it towards a destination equipment. This processing may for example include simple amplification, possibly combined with frequency or time translation. Alternatively, this processing may also include demodulation, error correction and re-modulation. Where the relay function terminates a link, a switching function will also be present in each relay equipment.
In such multi-hop wireless communications systems, the routing of indirect communications between the source equipment and destination equipment via at least one relay equipment is typically undertaken, using link state routing protocols. A single optimum path is generally chosen in accordance with the routing protocol between the source equipment and the destination equipment. To select an optimum path for a communication, the link state of all the links between the base station, end user terminal and the relay equipments have to be measured. Therefore, these protocols are inherently slow and cannot respond to rapidly changing propagation conditions. That is, propagation conditions may change too rapidly for the link state to be measured and for the link state measurements to be distributed around the system in a timely manner without excessive overhead. In addition whenever an optimum path changes, some link layer packets are left in buffers on the old optimum path, leading to fragmentation of higher layer packets, increased jitter and delay. Also, although the optimum path may be adjusted in response to traffic load, there is potential for added delay at the switching relay equipments.